1. Field of the Invention
The present invention relates to a spread-spectrum system, and more particularly to a spread spectrum diversity transceiver making use of code-division multiplexing, time-diversity, and error-correcting decoding, especially in digital radio transmission having difficulties in multipath fading.
2. Description of Related Art
Regarding a diversity system to be applied to digital communication, there has been disclosed a diversity system for preventing losses of data caused by fading in Japanese Patent Application Laid-open Publication No. 07-143101 (hereinafter, to be referred to as a first publication).
According to this first publication, after a predetermined length of transmission data has been coded into a code by an error-correcting encoder, the bit information of the code is transmitted simultaneously through a plurality of channels formed by transmitters of different frequencies and a transmission antenna.
At a receiver side, the bit information of the above one code is received simultaneously by a plurality of receivers through a receiving antenna and the received code information is subjected to error-correcting and decoding by an error-correcting decoder.
In such a radio transmission under channel fading, as described above, diversity receiving is usually necessary. Fading is broadly classified into frequency-nonselective (flat) fading and frequency-selective fading. The flat fading involves no occurrence of multipath propagation but directly causes variations in amplitude and phase of a receiving wave itself in the middle of its propagation. On the other hand, the frequency-selective fading involves an occurrence of multipath propagation and independently causes variations in amplitude and phase of each of a plurality of arrival waves due to multipath.
In the case of frequency-selective fading, a receiving signal becomes a combined wave of a plurality of multipath waves and, therefore, the received signal may have an inverse phase combination in a certain frequency depending on the status of phase variation. In other words, frequency-selective fading (a notch) occurs in the received spectrum.
On the other hand, in the case of flat fading, variations in the received signal level become a problem and the received waveform itself is not distorted. However, in the case of the frequency-selective fading by multipath, there occurs a distortion in the received waveform in addition to the variations in the received signal level.
For the above-described fading channel, there have been conventionally used diversity receiving and adaptive equalizer techniques. There are various types of system available for the diversity receiving and the adaptive equalizer technique. A spread spectrum technique which is considered to be effective for a multipath distortion will be explained below as an example of a prior-art technique.
The spread spectrum communication has so far been used for military purpose to achieve robust communication against interference waves.
In the case of multipath waves with a long delay time, however, correlation with a desired wave is reduced. In this case, if the spread spectrum technique is applied, a multipath wave is not correlated with a spreading code and is suppressed by a despreading operation. In other words, it can be said that the spread spectrum is a kind of an equalizer for regarding a multipath wave as an interference wave as well. However, a multipath wave with a short delay time has high correlation with the desired wave and this multipath wave cannot be suppressed easily by the despreading.
In this case, when there exists an inverse phase relation between the multipath wave and the desired wave, a signal level might be lowered or a fade-out could occur. In order to cope with such a fade-out, diversity receiving making use of non-correlation of a plurality of propagation paths becomes essential.
Referring to FIG. 1A, it is assumed that a transmitter 10 carries out transmission by using one non-directional antenna and a receiver 11 receives multipath propagating waves. Let us consider a model where multipath propagation occurs in which a wave radiated from the non-directional antenna of the transmitter 10 is propagated directly through a diversity path 12 and through diversity paths 13 and 14 including a reflection respectively.
FIGS. 1B-1D show variations of receiving electric field levels of the case where waves propagated through the diversity paths 12 to 14 respectively are received by the receiver 11. More specifically, variations in the received electric field of the waves that have propagated through each diversity path, that is, through the diversity path 13 in case of FIG. 7, through the diversity path 12 in case of FIG. 8, and through the diversity path 14 in case of FIG. 9, respectively, where the horizontal axis shows time and the vertical axis shows the levels of received electric fields or received power. In this case, since the propagation paths are different from each other in space, each fading becomes independent and the respective received signal levels vary as shown in FIGS. 1B to 1D.
In this model, the diversity receiving is performed by selecting or combining non-faded-out portions of the respective diversity branches to thereby reduce probabilities of fade-out.
The diversity like this is called a space diversity or a path diversity as the diversity utilizes non-correlation of the propagation paths. As a means for achieving this path diversity, an adaptive array using a plurality of antennas is usually employed. In other words, it is possible to achieve diversity combining by extracting a plurality of multipath arriving waves and performing maximum-ratio combining by the directional control of the adaptive array.
In general, however, the space diversity requires a plurality of antennas and is disadvantageous in cost. Particularly, in microwave communication, the number of antennas cannot be increased easily as the costs of antennas are high and a large-scale apparatus is required.
In order to improve the drawbacks of this space diversity, there has been disclosed a diversity system utilizing the spread spectrum code multiplexing and the time diversity in Japanese Patent Application Laid-open Publication No. 08-191289 (hereinafter to be referred to as a second publication).
This prior-art technique will be explained with reference to FIGS. 2A and 2B.
Referring to FIG. 2A, in a transmitter, transmission data is input to an error-correcting encoder 21 and the corded data is branched into N branches. The coded data is input to an interleaver 23(1) and is also input it interleavers 23(2)-23(N) through delay elements 22(1)-22(N-1) each providing the corresponding branch with a different delay.
The N-branch data with the delay step are independently interleaved by the interleavers 23(1)-23(N) and then the respective interleaved data are modulated by modulators 24(1)-24(N). Output signals of the modulators 24(1)-24(N) are sent to spectrum spreaders 25(1)-25(N), respectively. After having been spectrum-spread by the spreaders 25(b 1)-25(N), the N-branch signals of the same frequency band are output to a combiner 26.
The combiner 26 combines the N-branch signals into a code-multiplexed signal, which is converted into radio frequency by a radio transmitter 27. The radio-frequency signal is then transmitted as a radio wave through a transmission antenna 28.
Referring to FIG. 2B, in a receiver, a radio signal received by a receiving antenna 29 is input to a radio receiver 30, which converts the radio-frequency received signal into the spectrum-spreading frequency band. Then, the received signal is branched into N branches by a branch circuit 31. The N-branch received signals are input to despreaders 32(1)-32(N) corresponding to the N transmitting branches.
After have been spectrum-despread by the despreaders 32(1)-32(N) the respective signals are domodulated by demodulators 33(a)-33(N) and then the demodulated signals are deinterleaved by N deinterleavers 34(1)-34(N) in N branches. The received data obtained by the above process are output to delay elements 35(1)-35(N), respectively, so that the predetermined delay differences applied at a transmitting side are removed to produce N-branch received data with the same timing.
The N-branch received data passed through the delay elements 35(1)-35(N) are sent to a majority decision circuit 36. The majority decision circuit 516 has made a majority decision of the N-branch received data to produce error-corrected data. The error-corrected data is sent to an error-correcting decoder 37, which finally corrects errors of the error-corrected data to produce received data.
In the case of the conventional diversity system disclosed in the second publication, not only burst errors due to multipath fading or the like is randomized by interleaving, but also time diversity is carried out by delay processing, so that the channel can be improved even when fading occurs.
However, the combining of diversity branches is performed based on majority decision of the branches. For example, in the case of ten branches, if three branches show "1" of digital signal and the remaining seven branches show "0" of digital signal, then the majority decision circuit 36 decides "0".
However, there is a problem that if the number of branches showing "1" is five and the number of branches showing "0" is also five, a decision cannot be made as to which one is correct.
Further, as the error probabilities of the branches are random, there is also a possibility that a majority of branches take "0" despite a fact that "1" is a correct signal. In this case, the majority decision circuit 36 simply decides "0" of the majority and outputs this as correct.